1. Field of the Invention
The present invention is related to a formation method of a transparent electrode by a sputtering method. In addition, the present invention is related to an organic electroluminescence device and the manufacturing method.
2. Description of the Related Art
The field of application of a transparent conductive film is in optical communication, semiconductor laser, various display units, storage media, and apparatus for home use (a digital camera, a projector, a portable telephone, an optical lens, a mirror and a lamp). About a transparent conductive film, there are the following requests:
1. Stability in mass production such as yield improvement; and
2. Performance as a film in case of formation of a multilayer film.
An organic electroluminescence device is explained below. An organic luminescent layer is sandwiched between two electrodes. An organic luminescent layer emits light by electrifying between electrodes. An electrode in one side has to be transparent to take out light. It is proposed that a transparent conductive film comprising indium/tin oxides (ITO) is used as a transparent electrode (patent references 1, 2, 3 and 4).
As for the top emission type organic electroluminescence device, an electrode in the opposite side of a substrate is a transparent electrode. In a top emission type organic electroluminescence device, protection of a cathode and reduction of wiring resistance by forming a transparent conductive film on a metallic thin film are proposed. In addition, when a transparent conductive film is used as a cathode, for protection of an organic luminescent layer and reduction of an electron injection barrier, it is proposed that a buffer layer is inserted between an organic luminescent layer and a transparent conductive film.
Evaporation method, assist evaporation method by plasma and ion beam, ion plating method and ion beam sputter are used mainly as a manufacturing method of a transparent conductive film. In addition, a wet method such as a sol/gel method or a spray method can be used. On the other hand, a sputtering method is used in a mass production apparatus used for thin film manufacturing process of a semi-conductor, a flat panel display or an electronic component or the like.
A sputtering method is widely used as a method suitable for mass production from the following reasons:
1. A deposition rate or a film composition is stable; and
2. Uniform film formation to a substrate of large area is possible.
In addition, from the following reasons, a sputtering method is the mainstream method:
1. Uniformity of film thickness, conductivity and transparency is high; and
2. Fine etching property is good.
Characteristic of a sputtering method is described below.
When energy of the particle which is incident on a substrate is equal to or more than a 50 eV level, by the following phenomenon, problems such as impurity pollution or roughness of thin film generally occur:
1. A particle gets in the substrate;
2. Atom comprising the substrate is sputtered; and
3. Defect of a substrate occurs.
On the other hand, in methods such as evaporation method, film formation is performed only by thermal energy. Therefore, energy of an incident particle is at 0.1 eV level. Therefore, sufficient migration in a substrate surface is unable. Therefore, deposited film by particle adhesion is not dense. In addition, bonding strength of substrate-film interface is low and the bonding is unstable.
Energy of a sputter particle deposited on a substrate is much bigger than energy of a particle in a vacuum evaporation method. In the case of an evaporation method, particle energy is at 0.1 eV level. In the case of a sputtering method, particle energy is at 600 eV level. Therefore, when film formation on an organic thin film is performed by a sputtering method, break of molecular structure of a film occurs by scattering/crash of recoil Ar plasma, γ electron, the target particle or the like which is a high energy particle. Therefore, light emission performance of an organic luminescent material may fall.
On the other hand, when a metal mask is used in a sputtering method, and an electrode pattern is formed, from the following phenomenon, thermal expansion of a mask occurs and a mask may bend:
1. Radiant heat of a target surface by confined plasma; and
2. The secondary electron which is incident on the mask surface by end loss of plasma.
TABLE 1ThermalexpansionMagneticMaterialCompositioncoefficientproperties42AlloyFe—42Ni4.6e−6/° C.ParamagnetismSUS316Fe—18Cr—8Ni—Mo15.9e−3/° C. NonmagnetismSUS304Fe—18Cr—8Ni17.3e−6/° C. NonmagnetismInvarFe—Ni3.5e−6/° C.KovarFe—Ni—Coequivalentto invarSuper—1.0e−6/° C.Invar(MA-INV36/MA-S-INVER)
Material of low thermal expansion is used for a mask material with respect to the problem, and a bend of a mask is suppressed. For example, austenitic stainless steels and invar materials as shown in TABLE 1 are used.
Patent reference 1: Japanese Patent Laid-Open No. 2003-901158 Official Gazette
Patent reference 2: Japanese Patent Laid-Open No. 2001-250678 Official Gazette
Patent reference 3: The 2,850,906th Patent Official Gazette
Patent reference 4: Japanese Patent Laid-Open No. 2005-68501 Official Gazette
Patent reference 5: Japanese Patent Laid-Open No. 2001-176670 Official Gazette
In manufacture of a top emission type organic electroluminescence device, establishment of a transparent electrode film formation technique having the following characteristic is important:
1. Wiring resistance is low; and
2. Visible light transparency of wiring resistance is high.
The present invention uses a sputtering mask made of material of low thermal expansion and high electric insulation as a mask of forming transparent electrode pattern. For example, aluminum nitride ceramics (AlN) is used.
For a material of the mask, a material which is superior to machinability and is light-weight is desirable in order to form a highly minute pattern in a thin sheet.
In a top emission type organic electroluminescence device, a metal electrode, an organic thin film and a transparent electrode is layered in this order on a glass substrate. Color filter can be affixed to a sealing substrate side of this top emission type organic electroluminescence device. In addition, when a top emission type organic electroluminescence device is used as an active matrix drive display unit, light is not obstructed by a driving circuit in a substrate. Therefore, incrementation of open area ratio can be expected.
In a sputter process, an electrode material is an insulating material, and RF magnetron sputtering is used. Because RF magnetron sputtering is high energy process, in the case of film formation on an organic thin film, film formation in an extended period by low power must be performed. Because recoil Ar plasma, γ electron and an accelerated target particle collide to an organic thin film, an organic thin film can be damaged.